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Amnesty International has welcomed the release of a retired high-ranking military officer in Morocco, who was sentenced to a 12-year prison term after he wrote to the country's monarch complaining about the treatment of former air force pilots.

Kaddour Terhzaz was freed from Salé Prison near the capital city Rabat on Wednesday afternoon, after he had been granted a pardon by Morocco's King Mohamed VI.

The 73-year-old former colonel had been imprisoned since his conviction by a military court in November 2008. The court found him guilty of threatening Morocco's "external security" through divulging a secret of "national defence".

He had been held in solitary confinement since November 2009.

"While Kaddour Terhzaz's release is welcome news, he should not have been imprisoned in the first place for simply writing a letter," said Amnesty International.

"He was convicted and sentenced in a trial that did not meet international fair trial standards and has spent over two years in prison simply for exercising his right to freedom of expression."

Kaddour Terhzaz's conviction was based on an undated letter he sent to King Mohamed VI, calling on him to improve the situation of former pilots who had been held captive by the Polisario Front, which runs a self-proclaimed government in exile in the Tindouf refugee camps in Algeria and calls for the independence of Western Sahara, a territory annexed by Morocco in 1975.

He lamented that the pilots had not been treated with due respect and appreciation since their release.

He also gave the letter to a former Moroccan pilot and captive of the Polisario Front Ali Najab, who was active in lobbying for better treatment in Morocco of former prisoners of war.

In his letter, Kaddour Terhzaz claimed that Moroccan planes were not equipped with anti-missile systems at the time of the armed conflict between Morocco and the Polisario Front from 1975 to 1991.

The court ruled this to be confidential military information and Kaddour Terhzaz was sentenced to 12 years imprisonment.

Amnesty International believed that the content of the letter did not represent a genuine threat to national security, particularly as a ceasefire has been in place since 1991.

The organization has been campaigning for the release of Kaddour Terhzaz for the last 15 months.

Talking to Amnesty International after his father's release, Kaddour Terhzaz' son said: "Amnesty International was the first to trigger and highlight Kaddour Terhzaz's case, denouncing his arbitrary detention. You have followed the case constantly, with no interruption for asking his release. Your first report, your urgent actions, your web action... I would like to thank you a lot for everything and I hope AI continues the great work to fight against injustice all around the world."